1. Field of the Invention
The present invention relates to a method of producing single-crystal silicon carbide.
2. Related Arts
In recent years, a single-crystal substrate of silicon carbide has been developed as a semiconductor substrate for a semiconductor device such as a transistor, a diode or the like, which can withstand to high voltage and is operable with large electric power. Methods known in the art for producing the single-crystal substrate of silicon carbide are the Acheson method, the Lely method, the sublimation-recrystallization method (i.e., the modified Lely method) and the like. Among these methods, the sublimation-recrystallization method is conventionally adopted because it is advantageous to the growth of silicon carbide single-crystal substrate having a large area and high quality.
As disclosed in JP-B2-63-57400, in the sublimation-recrystallization method, silicon carbide source material held in a graphite crucible is heated for sublimation, whereby single-crystal silicon carbide is grown on a silicon carbide seed substrate made of single-crystal silicon carbide. The silicon carbide seed substrate is disposed opposite to the silicon carbide source material in the same graphite crucible. Thus obtained single-crystal silicon carbide has a large area and the polytype thereof is controlled. Therefore, it is suitable for the semiconductor substrate. Further, on the single-crystal substrate of silicon-carbide, a silicon carbide single-crystal layer having the different conductivity type or the different carrier concentration from the single-crystal substrate can be epitaxially grown by a liquid phase epitaxial (LPE) method or chemical vapor deposition (CVD) method.
However, the diameter of the silicon carbide single-crystal substrate obtained by the above-mentioned method is only approximately 1 inch. Therefore, it is needed to make the diameter of the-single-crystal silicon carbide substrate larger in order to mass-producing the semiconductor device such as the transistor or the like using the substrate.
Silicon carbide has many polytype structures each of which has a different crystal structure. These polytype structures are classified into an .alpha.-type and .beta.-type. Polytype structures of the .alpha.-type have crystal structures belonging to a hexagonal system and rhombohedral system. The hexagonal system is further classified into a 6H-type, 4H-type and the like on the basis of the number of laminated lattice planes within one cycle in the crystal structure. The rhombohedral system is further classified into a 15R-type, 21R-type and the like in the same way. A polytype structure of .beta.-type has a crystal structure belonging to a cubic system having only a 3C-type.
The electrical properties of the silicon carbide single-crystal substrate differ depending on the polytype structure. Therefore, when a specified semiconductor device utilizing a silicon carbide single-crystal substrate is produced, the polytype structure of silicon carbide is chosen on the basis of the required electrical properties of the semiconductor device. Further, the electrical properties of the silicon carbide single-crystal substrate differ depending on the crystal plane of the surface of the substrate. In a case that a semiconductor element having a geometrical shape such as a transistor or the like is formed on the silicon carbide single-crystal substrate, symmetry property of the silicon carbide crystal structure is also important. Concerning the .alpha.-type silicon carbide single-crystal substrate, when the surface of the substrate has a (0001) plane, the symmetry property is maximum. Accordingly, it is suitable as a substrate for the semiconductor device. The .alpha.-type silicon carbide single-crystal substrate is especially effective to improve the characteristics of the semiconductor device which can withstand to high voltage and to which large electric power is applied. Therefore, it is desirable that the .alpha.-type silicon carbide single-crystal substrates having a large diameter and high quality are produced in large quantities at low cost.
As a conventional technique to increase the diameter of the silicon carbide substrate, it is known that the Acheson crystal shaped and ground in advance can be used as a seed crystal to produce the silicon carbide single-crystal substrate thereon in the sublimation-recrystallization method. The Acheson crystal is single-crystal silicon carbide incidentally obtained in a process for producing silicon carbide abrasives in the Acheson furnace.
It is, however, difficult to produce the Acheson crystal having a large size. Therefore, JP-A-6-48898 proposes a method, in which silicon carbide is grown on the seed crystal having a small size in the beginning and is repeatedly grown so that the size of the single-crystal silicon carbide gradually becomes large.
JP-B2-1-38080 proposes another method. In the method, a .beta.-type silicon carbide single-crystal layer is epitaxially grown on a silicon substrate, and then the silicon substrate is removed. Thereafter, an .alpha.-type silicon carbide single-crystal layer is grown on the .beta.-type silicon carbide single-crystal layer by a CVD method. The method proposed in JP-B2-1-38080 has a possibility capable of producing the .alpha.-type silicon carbide single-crystal substrate having a large diameter.
Further, "Journal of Crystal Growth 99", 1990, PP. 278-283 reports another method in which a hexagonal single-crystal silicon carbide is grown on a (100) cubic silicon carbide layer by the sublimation-recrystallization method. The (100) cubic silicon carbide layer functions as a seed crystal. This method also has a possibility capable of producing the .alpha.-type silicon carbide single-crystal substrate having a large diameter.
However, the method proposed in JP-A-6-48898 involves a complicated process for growing silicon carbide repeatedly, thereby resulting in the rise in cost. Further, the diameter of the silicon carbide substrate is limited to only 2 to 3 inches at most in the present laboratory level.
In the method proposed in JP-B2-1-38080, the .alpha.-type silicon carbide single-crystal layer can be grown so as to have a required diameter. However, the .alpha.-type silicon carbide single-crystal layer is an epitaxial layer formed by the CVD method. Because of this, the thickness of the .alpha.-type silicon carbide single-crystal layer obtained in this method is 2 .mu.m at most, and it is difficult to produce bulk single-crystal silicon carbide having a thickness of more than 300 .mu.m required as a substrat. Further, in the method proposed in "Journal of Crystal Growth 99",1990, PP. 278-283, although it has a possibility to produce bulk single-crystal silicon carbide, it is difficult to produce single-crystal silicon carbide having a (0001) plane. The surface of the single-crystal silicon carbide has a (0114) plane.